Rare earth magnet question

I have been reading about searching for meteorites. I live in a good area
for it, with lots of open desert and few plants. Lots of sand dunes, and

lots of sandy washes.
I want to make a device that I can haul behind my ATV, like a rolling
magnetic nail sweeper. However, I want to make one on steroids with six
inch tires, and a row of very powerful rare earth magnets. I want it to be
three feet wide. I want a VERY powerful rare earth magnet about every six
inches. I want it to pick up meteorites and metal from at least three
inches, which would be the axle height.
Anyone have a clue as to what sized magnets I should be looking at? There's
millions of these magnets on ebay, but I'd just like to make one big mondo
one and see if it's going to work.
Help appreciated.
Steve

Steve Nooooooo!
Wind yer own electromagnet so you can turn it off when your ATV goes
'wheels up' on locating a chunk of railroad. If you don't feel like
winding, just bolt a bunch of A/C clutch coils facing downwards:

Indeed. Lots of road maint guys build rigs with electromagnets..so they
can dump the picked up items when the rig is full.
Unless you can pull that big chunck of scrap off that rare earth magnet
with a plastic crowbar.....
Gunner
A human being should be able to change a diaper, plan an invasion,
butcher a hog, conn a ship, design a building, write a sonnet,
balance accounts, build a wall, set a bone, comfort the dying,
take orders, give orders, cooperate, act alone, solve equations,
analyze a new problem, pitch manure, program a computer,
cook a tasty meal, fight efficiently, die gallantly.
Specialization is for insects. Lazarus Long

Good points have been made about an electromagnet instead of permanent
magnet. Picking up stuff from 6" takes a fairly mighty unit, I think
it'd end up by the time you've got the power supply, switchgear and
magnet together you might have quite a load, and anything of respectable
voltage makes a heck of an arc when you open the relay. 240v DC is
impressive..
Another approach might be to get a magnet that's made to hang from the
bumper of a yard truck, check out your local truck terminal to see an
example. The ones I've seen are about 48" wide, they pick up debris from
a paved surface pretty well and probably come in different sizes.
How about a permanent magnet roller with a belt over it? Let the thing
roll on the ground, run the belt over a small roller above/behind the
magnetic roller, and a hopper under the belt- sort of like a lawn
sweeper. Look up magnetic conveyor roller and magnetic separator, you'll
see what I'm talking about. Notice some belting has paddles or ridges to
help carry debris away from the magnet, otherwise it just slides on the
belt.
John

Steve, I have plenty of tiny neodymium magnets, and know that they are
fun, but they are also so strong that pulling them apart is not
easy. Try buying a 1x1x1" magnet and stick it to a steel block (try to
make sure that it does not slam into the steel or else it may
break). Then pry it off. You will be able to pry it off, but it will
be difficult. Now imagine that you have a magnet as big as you
described!
i

We are desert rats. We have found all sorts of things out there from
pioneer junk to arrowheads to WWII ordinance. And all the time, I could
have been dragging one of these, and snagging a meteorite here and there.
We're in a good spot with lots of flat areas and few plants. I'll keep
everyone posted as it goes.
Steve

Would be interesting. Effects of old projectiles jumping and slamming
into neodymium megnets may also be interesting.
A long time ago we were working in a field and my friend found a WWII
mortar shell.
Realistically, though, magnets' pulling power declines very quickly as
a function of distance, so even very strong magnets may not be able to
pull out something buried a couple of inches in the sand. I would
experiment on a small scale first.
i

We have found lots and lots of WWII ordinance. Mostly 50 cal. BMG casings.
I don't know if they are metallic, as they are all stored away now pending
moving. The bullet does have a steel armor piercing core. We do find live
rounds occasionally.
Steve

Rare Earth magnets store a lot of magnetic energy in a
small volume but their normal shape is such that the field
strength falls off very rapidly with distance. It's a complex
subject but VERY roughly speaking the main part of the useful
pickup field strength is around a volume centred on a semicircle
joining N pole to S pole. For your 3" working distance the magnet
length should be in the region of 6" !!!.
However this is only necessary if you aim to reach a large
fraction of the possible maximum flux density. If you are content
with somewhat lower values the effective magnet length can be
increased by suitably placed pole pieces.
The very simplest arrangement would be a series of short
bar magnets and pole pieces producing a
N-S-N-S-N-S........ field.
Pole pieces could be 6" long lumps of mild steel of the
same cross section as the magnet and.arranged as below.
(NS or SN is a magnet, PP is a pole piece, 1/2PP is half length)
1/2PP-NS-PP-SN-PP-NS-PP-SN-1/2PP
However this would be pretty inconvenient as most of the
pickup would gravitate towards the rare earth elements. This
would make it difficult to remove and probably damage the
magnets, It's better to convert the series of artificial "bar"
magnets to a series of "horseshoe" magnets. Pickup then sticks to
the ends of the horseshoe extensions and is safely distanced from
the expensive magnets.
The extensions that form the horseshoe shape should be
mild steel plates typically 4" to 6" long, the same width as the
6" long pole pieces but about 1/4 to 1/3 the cross sectional
area and located at the centre of each 6"pole piece.
For a test rig, make up a single horseshoe from one
magnet,two 3" pole pieces and two 5" extensions mounted at the
end of the 3" pole pieces. Buy a pair of large magnets and try
them out in the combinations
1 magnet
2 magnets side by side NN
SS
2 magnets on top of each other.NSNS
This will tell you whether a larger magnet area or larger
magnet thickness is the best way to go. Because of the very large
leakage flux in this arrangement I would expect largest magnet
area to be the best choice.
For the two magnets side by side test it is not necesary to
double the cross section of the pole pieces - just sandwich the
two magnets between two plates (1/4"++ thickness) and insert the
assembly between the 3" pole pieces. The magnets will object to
being same polarity side by side and you may have to temporarily
glue them in position.
You could also experiment with the 3" pole piece length.
Increased length reduces the useful field strength but the
reduced field is effective over a longer distance.
This is not the only possible arrangement. The basic
requirement is to have about 6" of pole piece in series with the
rare earth magnet. The pole piece can be split and the magnet
located at any convenient split position. One alternative
arrangement is a single pole piece bar extending the width of the
vehicle. At 6" intervals a magnet is located between the lower
side of the bar and a downward extension pole piece.
These arrangements should produce the right sort of field
shape but 3" is a very long working distance and it may be
difficult to achieve sufficient pickup power.
Jim

I once had a welder that had a magnet that was the size of a shoebox. We
used it to keep tanks from rolling around in the back of a truck. (dumb, I
know) He ran of with some stripper and left that and some other gear.
(actually went missing)
It was a Mongo magnet, and I wish I had not been parted from it. It would
be the perfect thing. Mount it on a 3' x 3" x 1" piece of plate, and the
whole thing would be a magnet.
Steve

Magnetic force varies with the square of the distance.
So if you want the magnet to pull one pound at 3"
then it will pull
9 lbs at one inch.
900 lbs at 0.1 inches
90,000 lbs at 0.01" inches.
Its not going to happen with any magenet you can physically lift.
And if you can physically lift it you would never ever get stray metal unstuck.
Sorry to be a spoil sport, but the physics just don't work.
Paul

While magnetic field falls off in an inverse-cube relation with
distance, apparently magnetic force between a magnet and another
object falls off even more rapidly. Eg, inverse 4th power between
magnets, inverse 7th power between a magnet and a bit of steel.
At least that's what the page
http://isaac.exploratorium.edu/~pauld/activities/magnetism/forcebetweenmagnets.html
says, near the end.

After reading all the replies so far (Mon 1603) I havent seen any
discussion of "pole peices" or the magnetic path. I think you can
do a lot with less magnet strength by aranging the rest of the
magnetic path to minimize the air gap. As is done in some magnetron
magnets. Too complex a subject to try to define here. :-)
...lew...

Very interesting discussion, on these rear earth magnets.
Now, I have a question not exactly along the lines of the discussion, but
pehaps at a tangent.. A friend of mine has told me that these magnets can
be wound with a coil, as in and an electromagnet, with amazing results to
their apparent strength, to increase or diminish it. Is this so?
Thank you,
Flash

As many others have pointed out, direct use of big magnets is not going
to be practical.
What will work is a large pulse induction metal detector. meteorites of
any size are going to give a huge signal.
One can purchase such units, or build them. for more information than is
healthy, go to:
<http://www.gi.alaska.edu/~jesse/treasure/misc/howdetector.html
and <http://www.findmall.com/list.php?34 (PI Technology Forum).
Joe Gwinn

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